Charge-transport molecular and polymeric materials are semiconducting materials in which charges can migrate under the influence of an electric field. These charges may be present due to doping with oxidizing or reducing agents, so that some fraction of the transport molecules or polymer repeat units is present as radical cations or anions. More usually, charges are introduced by injection from another material under the influence of an electric field. Charge-transport materials may be classified into hole- and electron-transport materials. In a hole-transport material, electrons are removed, either by doping or injection, from a filled manifold of orbitals to give positively charged molecules or polymer repeat units. Transport takes place by electron-transfer between a molecule or polymer repeat unit and the corresponding radical cation; this can be regarded as movement of a positive charge (hole) in the opposite direction to this electronic motion. In an electron-transport material, extra electrons are added, either by doping or injection; here, the transport process includes electron-transfer from the radical anion of a molecule or polymer repeat unit to the corresponding neutral species. In addition, some material—ambi-polar materials—may transport both holes and electrons.